The present invention relates to a miniature portable radio unit with a strip antenna and, more particularly, to its antenna and the associated part.
In FIG. 1 there is shown a conventional portable radio unit of this kind, which has a radio circuit 12 enclosed in a housing 11 and a whip type strip antenna 13 put up on the outside of the housing 11. The antenna 13 is connected to the radio circuit 12 via a matching circuit 14 in the housing 11. In general, the matching circuit 12 is needed except when the antenna 13 has an electrical length nearly equal to one-fourth the wavelength at the working frequency of the radio unit.
To obtain an excellent radiation pattern performance, the electrical length of the antenna 13 is preferably be about one-half of the working wavelength. The reason for this is that since a standing wave of a half-wave length gets onto the antenna element, the antenna current is reduced to zero at the site of attachment of the antenna element to the housing and the antenna is essentially free from the influence of the housing. In the conventional portable radio unit, however, the feeding point is at the site of attachment of the antenna element to the housing as referred to above; therefore, it is necessary that feeding to an antenna element whose electrical length is set to the half-wave length be done at a very high impedance--this requires the matching circuit 12. Since the matching circuit 14 is made up of a coil and a capacitor which are lumped parameter elements, their resistance causes a loss, and hence the use of the matching circuit impairs the antenna efficiency about 0.5 to 2 dB.
With the conventional portable radio unit, the above-noted antenna structure provides an excellent radiation pattern almost insusceptible to the influence of the housing but has the defect of decreased efficiency.
Moreover, the use of such a strip antenna makes it difficult to obtain a wide band characteristic. To solve this problem, it has been proposed to modify the matching circuit or load a coil on the antenna element at its tip or intermediate portion, but either method introduces complexity in the construction of the portable radio unit.
Furthermore, in the case where the whip antenna is the retractable type, mismatching occurs at the feeding point when the antenna is at its fully retracted position in the housing, and hence it hardly functions as an antenna, resulting in the gain being seriously impaired.
In view of the above, there has been proposed such a portable radio unit as shown in FIG. 2 (Japanese Patent Laid-Open No. 120902/92). In FIG. 2, reference numeral 15 denotes a dielectric plate which can be retracted down into and extended or projected out upwardly from the housing 11. The dielectric plate 15 has on one side a centrally-disposed strip conductor 16s extending lengthwise thereof and on the other side a wider grounding conductor 16e extending in opposing relation to the strip conductor 16e. The strip conductor 16s and the grounding conductor 16e constitute a micro strip line 16. Reference numerals 17a and 17b denote strip conductors connected to both ends of the grounding conductor 16e at the side opposite from the housing 11 and extending along both marginal edges of the dielectric plate 15. The electrical lengths of the strip conductors 17a and 17b are set to a quarter of the wavelength of the working frequency. Reference numeral 18 denotes a coiled conductor connected at one end to the strip conductor 16s at the side opposite from the housing 11. The electrical length of the coiled conductor 18 is also set to a quarter of the wavelength of the working frequency.
When the feeding point is located between the connection point of the coiled conductor 1 18 and the strip conductor 16s and the connection point of the grounding conductor 116e and the strip conductors 17a and 17b, the antenna current becomes substantially zero at the site of attachment of the antenna element to the housing 11 and the antenna element is not seriously affected by the housing 11 as is the case with the half-wave dipole antenna. When the dielectric plate 15 lies at its fully retracted position in the housing 11, the inner ends of the strip conductors 17a and 17b and the grounding conductor 116e are shorted and the coiled conductor 18 projects out of housing 11, and hence only the coiled conductor 18 functions as an antenna.
With the dielectric plate 15 held at its fully extended position, the antenna operates on the same principle of operation as that of a vertical antenna of the type wherein the center or core conductor of a coaxial cable is projected out therefrom by a quarter-wave length and the outer conductor is folded back by the quarter-wave length to form a cylindrical sleeve. Thus, the antenna characteristic is excellent in a plane containing the surface of the dielectric plate 15 but in a plane perpendicular thereto and containing the strip conductor 16s the antenna characteristic is unbalanced and an unbalanced current flows into the housing 11, with the result that the radiation pattern is distorted or disturbed and an energy loss is caused and hence the antenna efficiency is cut down.
Furthermore, the strip conductor 16s and the grounding conductor 116e of the micro strip line 16 need to be formed on the opposite sides of the dielectric plate 15, i.e. separated by the dielectric plate 15; this poses a problem as a decrease in the thickness of the dielectric plate 15 provides increased loss. The width of the grounding conductor 116e is required to be at least approximately three times larger than the width of the strip conductor 16s, though dependent on the thickness of the dielectric plate 15, and the strip conductors 17a and 17b must be provided along both sides of the grounding conductor 16e. Thus, the conventional portable radio unit depicted in FIG. 2 has a shortcoming that the width of the dielectric plate 15 is large.
It is therefore an object of the present invention to provide a portable radio unit with an antenna which does not suffer serious distortion or variation of its radiation pattern, has high efficiency and retains a high gain even while the antenna is at its fully retracted position in the housing.
Another object of the present invention is to provide a portable radio unit with an antenna which provides a wide band characteristic with ease as well as the above-mentioned features.